Numerous applications exist in which a burner is required to provide a stable flame while being subjected to winds or draughts of widely variable direction and speed, and under highly turbulent, or gusting conditions. Examples include flares, camping stoves, ceremonial torches and pilot burners in boilers and other industrial applications.
Flame stability is commonly achieved by the generation of a flow recirculation or a vortex flow pattern, either in the wake of a bluff-body or within the “vortex breakdown” associated with strongly swirling flows. While such flame holders are very successful in the relatively well defined conditions that occur within industrial combustion systems, they usually require that the combustion air be introduced through the burner in a carefully controlled manner in order to generate the necessary flow recirculation. The size, strength and stability of the recirculating flow is usually influenced by cross draughts in the furnace, or in the case of a flare, by the wind. To overcome the problem of sensitivity to the direction of the wind or cross-draught, the ideal aerodynamic flame holder should produce a recirculating flow pattern which is
(1) independent of the direction of the wind or cross draught, and
(2) insensitive to sudden changes in speed or to wind gusts.
A limiting factor in flame stability is the propagation speed of the flame front. Flame speed is a function of the fuel type and the air/fuel ratio and the turbulence. For most hydrocarbon fuels, the flame speed in a laminar flow (i.e. laminar flame speed) is typically less than 0.5 m/s. Although it is possible to produce stable flames in turbulent flows where the mean flow speed is an order of magnitude higher than the laminar flame speed, the actual local flame speed is still limited by the laminar flame speed. In contrast, instantaneous wind speeds in gusting conditions readily exceed 20 m/s and can reach speeds of 100 m/s or more. Hence a further purpose of a flame holder is to provide an aerodynamic “shield” which protects the flame (or at least the root of the flame) from high speed wind gusts. The aerodynamic shield provides a zone in which the flow speed is limited to the range of values necessary for good flame stability.